1. Field of the Invention
This invention relates generally to meltblowing dies and specifically to a meltblowing die with a replaceable die tip, wherein the air-gap and set-back parameters may be adjusted as desired before replacement. In another aspect, the invention relates to a method of replacing the die tip of a meltblowing die.
2. Discussion of the Prior Art
Meltblowing is a process for manufacturing nonwoven products by extruding a molten thermoplastic polymer resin through a plurality of small orifices to form fine fibers while blowing converging sheets of air onto each side of the orifices to attenuate and draw down the fibers. The extruded fibers are blown onto a moving collector surface where they entangle in a random way to produce a nonwoven fabric or web. The newly formed web is directly withdrawn from the collector for further processing as desired. The overall process is carried out continuously. The webs may be further processed into a number of commercially important products such as filters, fabrics, and battery separators among others. Meltblowing dies are disclosed in U.S. Pat. Nos. 3,978,185, 4,818,463, and 4,986,743.
A key component of a meltblowing die is the die tip. The die tip is an elongate member having an outwardly protruding nosepiece of triangular cross-section terminating in an apex with a row of side-by-side orifices drilled through the apex. A polymer melt is forced through the orifices for extruding the polymer and discharged as side-by-side molten or semimolten fibers. The die tip is generally formed of high quality steel in a separate machining process and fastened to the face of the die body using bolts. Precise machining is required to achieve uniform polymer flow over the length of the row of orifices. The polymer is forced through the die tip orifices by applying very substantial pressure to the molten polymer inside the tip. The internal pressure induces tensile stress in the die tip which tends to concentrate near the apex of the tip and may cause the tip to fail in tension. U.S. Pat. No. 4,986,743 teaches a method for mounting the die tip on the die body with induced compression in the apex region to counterbalance the tensile stress, rendering the die tip more reliable in service. This patent is incorporated herein by reference.
Other important components of the meltblowing die are elongate plates referred to as air knives which, in combination with the die tip nosepiece, form converging air flows to attenuate and draw down the extruded fibers to microsized diameters. The air knives are generally thick elongate plates which have a longitudinal edge tapered to form a knife edge. Conventionally, two air knives are bolted to the die body on either side of the triangular nosepiece of the die tip. The tapered edges of the air knives are aligned with the confronting tapered surfaces of the nosepiece and spaced slightly therefrom to form two air flow channels converging at the apex of the nosepiece, so that the air flowing rapidly past the apex of the nosepiece tends to entrain the fibers being extruded therefrom, and draw the fibers down until they break. The air flow characteristics are determined by the shape and size of the passages formed between the die tip and the air knives, which are defined by parameters known in the art as the "air-gap", the spacing between the confronting surfaces of the triangular nosepiece of the die tip and the air knives, and the "set-back", the vertical distance between the tip of the nosepiece and the outer plane of the air knives. (As used herein, terminology such as "upward", downward", "vertical", and the like refers to the usual orientation of the die, in which the fibers are extruded downwardly; however, the invention is not to be so limited).
The air-gap and set-back determine the geometry of the converging air flow passages, and thereby influence the air flow properties and the degree of fiber-air interaction. Research has shown that controlling the air flow properties in relation to the polymer flow properties is important for achieving the desired degree of fiber attenuation and final fiber diameter. Research has further shown that the fiber diameter strongly influences the properties of the web such as filtration efficiency.
In typical die designs, the air-gap and set-back are adjustable for selecting the air flow characteristics, to obtain the desired fiber and web product. Meltblowing polymers with different compositions may require different air-gap and set-back. Large meltblowing dies are normally vertically oriented and discharge downwardly onto a moving collector surface such as a conveyor screen or rotating drum.
A common problem in meltblowing occurs when the die tip plugs or structurally fails requiring that the die tip be removed for cleaning or replacement. In a conventional meltblowing die, the die tip and air knives are separate components individually bolted to the face of a die body fixed to the extruding and air supply components. Typical meltblowing die structures are shown in U.S. Pat. Nos. 4,818,463 and 4,986,743. Removal of the die tip generally requires first that each air knife be detached from the die body to gain access to the die tip. In order to remove the air knives, the air knife bolts are removed while the air knives are supported by workers, by cranes or the like. When the bolts have been removed, the air knives may be lowered from the die face and withdrawn from the workspace. The die tip bolts are then removed while the die tip is similarly supported, and the die tip is lowered and withdrawn. For large dies this procedure may be hazardous due to the large weight of the air knives and die tip. For dies employing a large conveyor-type collector, it may not be feasible to move either the die or collector to facilitate replacement of a fouled or damaged die tip; this further complicates the procedure due to the restricted space between the die and collector, forcing the workers to disassemble the air knives and die tip while beneath the die, in some cases being obliged to stand on the collector. These difficult working conditions are further exacerbated by high temperature of the die assembly. More specifically, in typical operation, the die tip and air knives may be at temperatures above four hundred degrees Fahrenheit and the meltblowing line must be shut down for an extended period to allow for cooling before repairs can be made or die tip replacement effected.
Similar difficulties are encountered in the reverse installation procedure, which is further complicated by the necessity of adjustment of the air-gap and set-back after the die tip and air knives have been reattached. If conventional dies are employed, these adjustments must be carried out with the die tip and air knives mounted on the die body. Moreover, the adjustment process involves the steps of mounting the air knives in a first position with respect to the die tip, measurement of the air-gap and set-back, removal of the air knives and reassembly with the air knives at different positions, remeasurement of the air-gap and set-back, and like tedious and repetitive operations, all commonly carried out under very inconvenient working conditions. The result is that often the adjustments are hurried and not made correctly.